Zeolites are hydrated metal aluminosilicate compounds with well-defined (tetrahedral) crystalline structures. Because zeolite crystals (both natural and synthetic) have a porous structure with connected channels extending through them, they have been employed as molecular sieves for selectively absorbing molecules on the basis of size, shape, and polarity.
Volumes packed with zeolite crystals (for example, small zeolite crystals chosen to have size in the range from 0.2 mm to one quarter inch) have been employed in air (or other gas) and water filtration systems to selectively absorb contaminants from a flowing stream of water or gas.
U.S. Pat. No. 5,567,405, issued Oct. 22, 1996 (based on U.S. application Ser. No. 08/445,025, filed May 19, 1995), and U.S. Pat. No. 5,573,743, issued Nov. 12, 1996 (based on U.S. application Ser. No. 08/445,076), teach methods for producing zeolite crystals impregnated with one or more of sodium chlorite, acetic acid, phosphoric acid, and citric acid, and methods for producing chlorine dioxide by moving a fluid (e.g., air or water) relative to a bed of zeolite crystals impregnated with sodium chlorite, and moving the fluid relative to another bed of zeolite crystals impregnated with one of the following: phosphoric acid, acetic acid and citric acid. The two beds can be physically mixed together, or the fluid can flow sequentially through distinct first and second beds. These references also teach a method for filtering a fluid by producing chlorine dioxide in the fluid (in the manner described in this paragraph) and then absorbing the chlorine dioxide from the fluid.
U.S. patent application Ser. No. 08/704,086, filed Aug. 28, 1996, now U.S. Pat. No. 5,730,948 teaches a variation on the chlorine dioxide production method of U.S. Pat. No. 5,567,405, which includes the steps of moving a fluid through a first bed of impregnated zeolite crystals (impregnated with at least one of phosphoric acid, acetic acid, and citric acid) and then moving the fluid through a second bed of impregnated zeolite crystals (impregnated with sodium chlorite).
Chlorine dioxide (ClO.sub.2) is useful for killing biological contaminants (such as microorganisms, mold, fungi, yeast and bacteria) and for oxidizing volatile organic chemicals which can contaminate fluid.
It is known to produce chlorine dioxide by: activating a metal chlorite solution by adding an acid thereto, activating a powdered composition (or other dry composition) by adding water thereto, or preparing an activated dry composition which releases chlorine dioxide over time. U.S. Pat. No. 4,547,381 (issued Oct. 15, 1985) and U.S. Pat. No. 4,689,169 (issued Aug. 25, 1987) mention these three techniques for producing chlorine dioxide, and disclose in some detail one type of such an activated dry composition. They teach that this activated dry composition is a mixture of a "dry inert diluent," a metal chlorite, and a dry agent capable of reacting with the metal chlorite in a dry state to produce chlorine dioxide. The metal chlorite can be sodium chlorite, and the dry agent can be a "dry acid" such as granular citric acid. The inert diluent can be diatomaceous earth, sodium chloride, sodium silicate, disodium sulfate, or magnesium chloride, or a combination of two or more thereof. The mixture releases chlorine dioxide over time until the rate of chlorine dioxide release becomes low, and the patents teach that the mixture can then be agitated for "renewed generation" of chlorine dioxide.
However, an activated composition (such as that described in U.S. Pat. Nos. 4,547,381 and 4,689,169) is subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide before the time of intended use of the composition.
Similarly, the inventor has found that sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) are also subject to undesirable storage and shipping losses, due to outgassing of chlorine dioxide therefrom before the time of their intended use (e.g., before fluid is caused to flow through both a bed of the sodium chlorite-impregnated zeolite crystals and a bed of acid-impregnated zeolite crystals). Also, efforts to activate sodium chlorite-impregnated zeolite crystals (of the type described in referenced U.S. Pat. Nos. 5,567,405 and 5,573,743) with acid at low temperature (below 40 degrees Fahrenheit) may fail in the sense that they will not result in release of sufficient amounts of chlorine dioxide.
There are also disadvantages to use of conventional metal chlorite solutions (of the type mentioned in U.S. Pat. Nos. 4,547,381 and 4,689,169) to produce chlorine dioxide. For example, when such a conventional solution is activated (by adding an acid thereto) to release chlorine dioxide gas, it is difficult or impossible to control the rate of release of the chlorine dioxide gas. Overproduction of chlorine dioxide often results.
There are also disadvantages to use of conventional powdered or dry compositions of the type activated by adding water thereto to release chlorine dioxide (as mentioned, for example, in U.S. Pat. Nos. 4,547,381 and 4,689,169). Masschelein, in the book Chlorine Dioxide--Chemistry and Environmental Impact of Oxychlorine Compounds (published 1979 by Ann Arbor Science Publishers Inc., Ann Arbor, Mich.) at page 140, describes one such dry mixture comprising sodium chlorite, and a solid organic anhydride containing 2 to 20% of a desiccating product such as calcium chloride. When such a conventional dry composition is activated (by adding water thereto) to release chlorine dioxide gas, it is typically difficult or impossible to control the rate of release of the chlorine dioxide to achieve chlorine dioxide release rates useful for such applications as air or water filtration. Overproduction of chlorine dioxide often results.
Until the present invention, it was not known how to produce a stable, unactivated substance (either in liquid or dry form) which does not release significant amounts of chlorine dioxide until activated by exposure to an acid, and which releases chlorine dioxide at a useful (and controllable) rate when exposed to an acid (even at temperatures below 40 degrees Fahrenheit). Nor had it been known to use such a stable, unactivated substance to produce controlled release of chlorine dioxide for filtering air or water.